Carburized tungsten alloy article



United States Patent CARBURIZED TUNGSTEN ALLOY ARTICLE William W.Wellborn, Pittsburgh, Pa., assignor to Firth Sterling, Inc., Pittsburgh,Pa., a corporation of Pennsylvania No Drawing. Application August 28,1953, Serial No. 377,250

1 Claim. (Cl. 148-32) This invention relates to composite carburizedarticles and process of making the same. The invention is useful in theproduction of articles such as drawing and extrusion dies, rolls forrolling rod and wire and for parts in general which require abrasionresistance and resistance to impact.

Sintered carbide compositions comprising tungsten carbide or other hardmetal carbide bonded by binding metal such as nickel, cobalt or iron,are known to be useful as cutting tools, dies and abrasion resistingarticles due to their high abrasive quality. However, such hard metalcarbide compositions do not have sufiicient impact resistance for useunder certain conditions. In accordance with my invention, I providecomposite carburized bodies or articles which comprise an integrallybonded core and a carburized case. The core comprises by weight at least80% tungsten and the case contains tungsten carbide formed in situ. Thecore imparts excellent impact resist ance to the article and the caseexcellent abrasive qualities.

The process of making the composite article involves providing a heavymetal body containing by weight at least 80% tungsten, and thereafterheating the body at a carburizing temperature in contact with acarburizing agent which is a solid at the carburizing temperatureemployed until a carburized case of desired thickness is formed on thebody.

While the heavy metal body, before carburizing, may be constitutedentirely of tungsten, it is preferred that it contain both tungsten anda metal binder, the tungsten constituting from 80 to 97% by weight ofthe body and the binder from 3 to 20%. All proportions given herein areby weight. As binder metals for the tungsten, I may employ nickel,cobalt or iron, or a mixture of any of them. I may use copper or silveror both as binder metals but only in combination with nickel or cobaltor iron or all of them and where copper or silver are used, .they do notconstitute over /6 of the total weight of binder metals.

A preferred heavy metal composition contains tungsten, nickel andcopper, the preferred proportions being:

Heavy metal bodies which are to be carburized can be made in variousways. The powdered metals can be intimately mixed as by ball milling andthen hot pressed into the desired shape. Instead of hot pressing, themixed powders can be cold pressed and thereafter sintered. The hotpressing, cold pressing and sintering are well known 2,778,757 PatentedJan. 22, 1957 "ice steps in the production of bodies from powdered metaland therefore need not be described in further detail.

Another method of producing the heavy metal body which is to becarburized sebsequently is to produce a green compact of powderedtungsten, place the compact at the bottom of a crucible, then placepowdered nickel or a mixture of nickel and copper on top of the compactand heat the crucible until the nickel and copper have melted and flowedinto the interstices of the green compact. Another method is to pournickel or an alloy of copper and nickel on a green compact of tungstenpowder so as to cause the compact to become impregnated with the bindermetal.

However the heavy metal body is formed, it is then carburized to form acarburized case integrally bonded to the core. For example, if anextrusion die is to be made according to the present invention, a heavymetal body of suitable shape and size for the extrusion die is made inany of the ways which have been described. Thereafter, the die ismachined to suitable shape and size, which can be readily done since theheavy metal body does not contain any carbides but merely the metalsthemselves.

The die is packed in graphite, coke, charcoal or other suitablecarburizing agent which is a solid at the carburizing temperature to beemployed. Powdered graphite is a preferred carburizing agent andpreferably is 100 mesh or finer in size. It should be of high puritysince if it contains sulphur it is likely to attack the surface of theheavy metal body. The die is packed in graphite in such manner that allsurfaces which are to be carburized are in intimate contact with thegraphite and the die so packed in powdered graphite in a graphite boatis placed in a furnace such as a hydrogen atmosphere furnace or otherfurnace having provision for maintaining a neutral or reducingatmosphere in the furnace. The die is heated at a carburizingtemperature until a carburized case of desired thickness is formed onthe body. The carburizing temperature may be from 1800 to 2700 F., apreferred range being between 2000 and 2400 F. The time of carburizingwill vary according to the thickness of the carburized case desired andaccording to the particular temperature and particular carburizingagent. Using powdered graphite as the carburizing agent and atemperature of about 2200 F., it has been found that a die whose largestdimension is 1 inch can be carburized to produce a carburized case of0.030 inch in approximately 4 hours.

The invention has particular utility in the production of dies forextruding aluminum. Aluminum oxide, which is very abrasive, forms onaluminum billets when they are heated for the purpose of extruding them.A die made according to my invention resists the abrasive action ofthese oxides and, in addition, due to the core of the die which is notcarburized, resists the impact involved in carrying out the extrusion.It will be evident that my composite article having an integrally bondedcore and a carburized case provides greater impact strength than asimilar article made throughout its thickness of tungsten carbide bondedby the same binder metal. On the other hand, my composite article,because of its carburized case, which contains tungsten carbide,provides much greater wear resistance than an article made throughoutits thickness of tungsten metal bonded by the same binder metal.

I have found that in order to obtain the desired results in carrying outthe carburizing step, the carburizing agent must be one which is a solidat the carburizing temperature employed. It is not satisfactory toemploy simply a carburizing gas. For example, I have employed methanegas as the carburizing agent but have found it to be unsatisfactory. Itresults in blistering of the heavy metal body. It is believed that thisis due to premature melting ofthe nickel or other binder m'ctalwhere abinder metal is employed. Where a gas isernployed as the carburizingagent, this premature melting occurs before the carburizing agenthas hadan opportunity to produce a carburized case of the desired thickness.Itis' therefore critical in carrying out my method that the carburizingagent be one which is a solid at the carburizing temperature, ascontrasted with the use of a gas as the carburizing agent.

The invention is not limited to the preferred embodiment but may beotherwise embodiedor practiced Within the scope of the following claim,

mam:

A composite articlecomprising an integrally bonded core and a carbnrizedcase, said core comprising by weightab'out 80'to' 97% tungsten'and' 3 to"20%"'binder metal, said binder metal consisting substantially entirelyof at least one member of the group consisting of nickel, cobalt andiron and at least one member of the group consisting of copper andsilver, the copper and silver being from zero percent to a percent notover /2 the percent of saidbinder metal, saidfcase containing tungstencarbide formed in situ and bonded by said binder metal.

2,242,254 Man fie May 20, 1941

